Bombs exploded early Thursday at two U.S. drug firms, and another device was defused at a local branch of a U.S. bank in the sixth attack on an American firm within 24 hours, police said.

There was no claim of responsibility and no injuries in the pre-dawn blasts, which followed explosions and attempted bombings targeting American, Japanese and Hong Kong business interests.Police believe the attacks have been carried out by military extremists seeking to topple President Corazon Aquino by frightening away foreign investors and embarrassing the government abroad.